Published: Nov. 23, 2009
Updated: Nov. 15, 2010
By Angela Spivey
Although colon cancer has been well-publicized as the second leading cause of cancer deaths in the United States, only about half of the people who should get screened for the disease actually do.
It’s not hard to imagine why: colonoscopy, the current gold standard for screening, is no fun. The rigors of “bowel prep.” Sedation. An endoscope inserted into the colon. But in 1993 a less invasive option came on the scene -- "virtual colonoscopy," or CT colonography, which involves the same bowel prep as colonoscopy, but neither sedation nor scope.
"We insufflate the colon with carbon dioxide, and in a single breath-hold take a CT scan of the abdomen," says Erik Paulson, MD, chief of abdominal imaging at Duke.
"Then the study is over. After the procedure, patients can return to work."
Physicians at Duke offer CT colonography as a clinical option, participate in its development, and train physicians in its use. Some studies suggest that CT colonography is comparable with colonoscopy in terms of effectiveness for most patients, especially when weighed in terms of its comparative ease.
But it isn’t perfect; even the major organizations that promote colon cancer screening have not yet recommended it as the procedure of choice for routine screening for average-risk adults.
In 2008, in the first-ever joint guidelines for colon cancer screening, the American College of Radiology, the American Cancer Society, and the U.S. Multi-Society Task Force on Colorectal Cancer specifically included CT colonography among several recommended options for screening and prevention in average-risk adults.
These guidelines differ from those issued that same year by the U.S. Preventive Services Task Force, which express doubt about the widespread accuracy of CT colonography because most physicians still have little experience with it.
For some patients, the dueling guidelines won’t matter because of a practical issue -- payment. Medicare and Medicaid, as well as some insurance companies, still do not cover CT colonography for patients at average risk for colon cancer. Some private insurers do cover CT colonography, but coverage is variable.
Medicare and Medicaid pay for the procedure only for patients whose condition makes a standard colonoscopy riskier than usual, such as if they’re taking anticoagulants or can’t be sedated for some reason. It may be covered for patients who have had an attempted colonoscopy that wasn’t completed because of bowel blockage.
In addition to colon blockage being a reason for an incomplete colonoscopy and appropriate referral for CT colonography, patients with tortuous (twisty) colons may have an incomplete colonoscopy and be appropriate referrals for CT colonography.
Those rules aren’t likely to change soon. In a final decision released in May 2009, Medicare and Medicaid announced they would not cover CT colonography for routine screening. But some private insurance companies have begun paying for CT colonography for routine screening for patients 50 and older.
"That’s a big change," Paulson says. Multiple studies showing that CT colonography rivals colonoscopy are what have turned the tide.
Paulson points in particular to a multi-institutional trial published September 18, 2008, in the New England Journal of Medicine.
"That study showed that the sensitivity and specificity of CT colonography is competitive with colonoscopy," Paulson says.
In the study, 2,800 patients underwent CT colonography and then a colonoscopy, and the CT version identified 90 percent of patients with polyps or cancers that were 10 millimeters or more in diameter.
Some previous studies, including one at Duke in which Paulson was involved (published in Lancet in 2005), showed that while CT colonography was good at detecting actual cancers, it was not as good as colonoscopy at detecting polyps.
But Paulson says the technology has since made big leaps thanks to advances in bowel preparations, the three-dimensional technology used to interpret the scans, computer-aided detection software which increases the accuracy of interpretation, and the ability to label residual fecal matter in the colon so it doesn’t show up on the test.
He and other Duke researchers have studied the technique -- leading research including multi-institutional clinical trials, the causes of false-negative and false-positive interpretations, and evaluation of computer-aided detection software.
Duke Radiology has for the past five years offered CT colonography as part of its routine clinical practice.
"We have six radiologists in our department who are skilled and experienced at CT colonography," Paulson says. "We’re doing more of them now than we’ve ever done."
Duke gastroenterologist Joanne Wilson, MD, does think that less-invasive tests can increase screening rates.
"Definitely the biggest impact something like CT colonography will have is getting more people screened who are at average risk," Wilson says.
But she sees the technology as one that’s not ready to be widely implemented.
"CT colonography has promise, but there probably needs to be some further development of the technology," she says.
Also, many current physicians aren’t prepared to offer the procedure.
"One of the points raised in the literature is that radiologists who were trained just in standard CT would need to gain additional training in order to conduct and read CT colonographies," Wilson says.
"When new technology is introduced, there’s always a concern about how you’re going to train currently practicing physicians."
Wilson also points out that if alternative tests such as CT colonography or stool tests come back positive, the patient likely will have to have a colonoscopy anyway in order to remove or sample the lesion.
"The colonoscopy is both diagnostic in the sense that you can see polyps, and it’s therapeutic because you can take them out, or you can mark them or sample them. The final diagnosis of cancer is a histological diagnosis; you want to look at the tissue with the microscope," she says.
She also emphasizes that colonoscopy will remain the recommended test for patients at high risk for colon cancer -- those with a prior history of colon polyps and colon cancer and those with a family history of polyps and cancer.
Paulson acknowledges that colonoscopy is still the tried-and-true gold standard.
"There’s no doubt that colonoscopy is a great test," he says. "For many people it makes all the sense in the world. But as good as it is, it has some risk and requires sedation and is more invasive."
And, he says, while colonoscopy is a mature technology, the virtual version can be expected to continue to make technological leaps.
This article was first published in the Fall 2009 edition of DukeMed Magazine.